Cell membranes and transport

membranes enable..

cells to receive hormone messages and in chloroplasts they contain the light-absorbing pigments needed for photosynthesis

if phospholipids are mixed with water they form either..

ball like structures- micelles

sheet like structures- bilayers

structure of a micelle?

all hydrophilic heads face outwards into the water

they shield the hydrophobic tails from the water

in the middle of the ball, the tails point in towards each other, creating a hydrophobic environment inside the micelle.

structure of bilayers?

hydrophobic tails are also shielded from the water by the hydrophilic heads

the bilayer is about 7nm wide

membranes also contain proteins

fluid mosaic model?

the currently accepted model of membrane structure, proposed by Singer and Nicolson in 1972, in which protein molecules are free to move about in a fluid bilayer of phospholipid molecules

why is it "fluid"?

both the phospholipids and the proteins can move about by diffusion

phospholipid molecules move sideways in their layers, some protein molecules can move within the bilayer. Others remain fixed to structures inside or outside of the cell.

why is it "mosaic"

describes the pattern produced by the scattered protein molecules when the surface of the membrane is viewed from above

structure of the fluid mosaic model?

phospholipid tails point inwards, facing each other and forming a non polar hydrophobic interior. the phospholipid heads face outwards into the aqueous medium that surrounds the membranes

what increases the fluidity of the membrane?

the more unsaturated the phospholipid tails are, the more fluid the membrane. unsaturated fatty acid tails are bent and therefore fit together more loosely

some organisms which cannot regulate their own temperature such as bacteria, respond by increasing the proportion of unsaturated fatty acids in their membranes

what decreases the fluidity of the membrane?

the longer the tail, the less fluid the membrane.

as temperature decreases the membranes become less fluid

where are proteins found?

in the inner layer, outer layer or most commonly spanning the whole membrane (transmembrane proteins)

why do the proteins stay inside the membrane?

they have hydrophobic and hydrophilic regions.

the hydrophobic regions made from hydrophobic amino acids, are next to the hydrophobic fatty acid tails and are repelled by the watery environment either side of the membrane.

the hydrophilic regions are repelled by the hydrophobic interior of the membrane and therefore face into the aqueous environment inside or outside the cell, or line the hydrophilic pores which pass through the membrane

many proteins and lipids have..

short, branching carbohydrate chains, forming glycoproteins and glycolipids.

the carbohydrate chains face outside the membranes. cholestrol molecules are also found in the membrane

role of phospholipids in membranes?

because the tails of phospholipids are non polar, its difficult for polar molecules/ions to pass through membranes. membranes can therefore act as a barrier to most water soluble substances. this means that water soluble molecules and unwanted water soluble molecules cannot enter the cell

some phospholipids can be modified..

to act as signalling molecules

cholestrol?

a small, lipid-related molecule with a hydrophilic head and a hydrophobic tail which is an essential constituent of membranes, it is particularly common in animal cells and gives flexibility and stability to the membrane as well as reducing fluidity

where is cholestrol found?

they fit between the phospholipid molecules

cell surface membranes in animal cells contain almost as much cholestrol as phospholipid.

cholestrol is much less common in plant cell membranes ans is absent from prokaryotes

how does cholestrol strengthen membranes?

by getting in between the phospholipid molecules and reducing fluidity. Without cholestrol, membranes quickly break and cells burst open

the hydrophobic regions of cholestrol molecules help to prevent..

ions/polar molecules from passing through the membrane

important in the myelin sheath

leakage of ions would slow down nerve impulses

how do cells survive cooler temperatures?

at low temp. the phospholipid tails tend to pack closer together and cholestrol prevents this from happening too much.

maintains the correct fluidity of the membrane

Glycolipids, glycoproteins and proteins: receptor molecules?

receptor molecules can bind with particular substances at the cell surface.

signalling receptors are part of a signalling system that coordinated the activities of cells, they recognise messenger molecules like hormones and neurotransmitters.

a series of chemical reactions begin inside the cell when the messenger molecules binds to the signalling receptor

Glycolipids, glycoproteins and proteins: cell to cell recognition?

some glycolipids and glycoproteins act as cell markers or antigens, allowing cells to recognise each other.

the carbohydrate chains bind to complementary sites on other cells

cell to cell recognition is important in growth and development and for immune responses. each type of cell has its own antigen.

Glycolipids, glycoproteins and proteins: transport proteins?

provide hydrophilic channels or passageways for ions and polar molecules to pass through membrane

each transport protein is specific for a particular kind of ion/molecule. 2 types of transport protein

channel proteins

carrier proteins

Glycolipids, glycoproteins and proteins: enzymes

some membrane proteins are enzymes. eg. digestive enzymes found in the cell surface membrane of the cells lining the small intestine

these catalyse the hydrolysis of molecules such as disaccharides

Glycolipids, glycoproteins and proteins: cytoskeleton?

some proteins on the inside of the cell membrane are attached to a system of protein filaments inside the cell known as the cytoskeleton.

helps to maintain and decide the shape of the cell

may also be involved in changes of shape when cells move

Glycolipids, glycoproteins and proteins: other roles?

proteins play important roles in the membranes of organelles

eg. in the membranes of mitochondria and chloroplasts they are involved in the processes of respiration and photosynthesis

cell signalling?

the molecular mechanisms by which cells detect and respond to external stimuli, including communication between cells

signalling pathways can be...

electrical or chemical

involve a wide range of molecules such as neurotransmitters and hormones

ligand?

a biological molecule which binds specifically to another molecule, such as a cell surface membrane receptor during cell signalling

the first part of the pathway involves which 3 main stages?

a stimulus causes cells to secrete a ligand eg. glucagon

the ligand is transported to the target cells. signalling molecules are usually relatively small for easy transport. (transport system is blood system for hormones)

the ligand binds to cell surface receptors on the target cells. the receptors are protein molecules located in the cell surface membrane

what happens to the cell surface receptor?

it is a specific shape and recognises the ligans. only cells with this receptor can recognise the ligand.

the ligand brings about a change in the shape of the receptor

the receptor spans the membrane, so the message is passed to the inside of the cell. Changing the shape of the receptor allows it to interact with the next component of the signalling pathway, so the message gets transmitted

transduction?

occurs during cell signalling, is the process of converting a signal from one method of transmission to another

what is the next component in the signalling pathway?

a 'G protein"

acts as a switch to bring about the release of a 'second messenger'.

what is the second messenger?

a small molecule which diffuses through the cell relaying the message

it typically activates an enzyme, which in turn activates further enzymes increasing the amplification at each stage. finally, enzymes are produced which bring about the required change in cell metabolism.

the stimulation of one receptor molecule results..

in many second messenger molecules being made in response. this represents an amplification of the original signal.

the second messenger typically activates an enzyme, which in turn activates further enzymes increasing the amplification at each stage. finally, enzymes are produced which bring about the required change in cell metabolism.

signalling cascade?

the sequence of events triggered by the G protein

3 other ways a receptor can alter the activity of a cell?

opening an ion channel, resulting in a change of membrane potential

acting directly as a membrane bound enzyme

acting as an intracellular receptor when the initial signal passes straight through the cell surface membrane

hydrophobic signalling molecules can..

diffuse directly across the cell surface membrane and bind to receptors in the cytoplasm or nucleus.

direct cell to cell contact is another mechanism of signalling

5 basic mechanisms by which exchange is achieved?

· Diffusion

· Facilitated diffusion

· Osmosis

· Active transport

· Bulk transport

diffusion?

the net movement of molecules or ions from a region of higher concentration to a region of lower concentration down a concentration gradient, as a result of the random movements of particle.

what is the random movement caused by?

the natural kinetic energy (energy of movement) of the molecules or ions.

as a result of diffusion..

As a result of diffusion, molecules or ions tend to reach an equilibrium situation where they are evenly spread out within a given volume of space.

how do respiratory gases and water cross membranes by diffusion?

They are uncharged and non-polar, and so can cross through the phospholipid bilayer between the phospholipid molecules. Water molecules, despite being very polar, can diffuse rapidly across the phospholipid bilayer because they are small enough.

how do hydrophobic molecules cross membranes?

because the interior of the membrane is hydrophobic.

the rate at which a substance diffuses across a membrane depends on..

· steepness of the concentration

· gradient

· temperature

· the nature of the molecules or ions

· surface area.

the steepness of the concentration gradient?

The steeper the concentration gradient of a substance across a membrane, the faster the rate of diffusion of that substance.

The steepness of the gradient is the difference in the concentration of the substance on the two sides of the membrane.

If there are more molecules of the substance on one side of the membrane than on the other, there will be a net movement of molecules from where there are more to where there are fewer

net movement?

although molecules move in both directions, more will move one way than the other depending on the gradient

Temperature?

Molecules have more kinetic energy at high temperatures and move faster so diffusion is faster

the nature of the molecule of ions?

Large molecules require more energy to get them moving than small ones do, so large molecules tend to diffuse more slowly than small molecules. Non polar molecules, such as glycerol, diffuse much more easily through cell membranes than polar ones, because they are soluble in the non-polar phospholipid tails.

the surface area across which diffusion is taking place?

The greater the area of a surface, the more molecules or ions can cross it at any one moment, and therefore the faster diffusion can occur. The surface area of cell membranes can be increased by folding. The larger the cell, the smaller its surface area in relation to its volume. Volume increases much more rapidly than surface area as size increases.

the surface area to volume ratio?

The surface area: volume ratio decreases as the size of any three-dimensional (3D) object increases.

why is diffusion only effective over very short distances?

This results in a limit on the size of cells, because the time taken to travel any distance by diffusion increases much faster than the distance does

facilitated diffusion?

the diffusion of a substance through a transport protein (channel protein or carrier protein) in a cell membrane; the protein provides hydrophilic areas that allow the molecule or ion to pass through the membrane, which would otherwise be less permeable to it

Why cant Large polar molecules, such as glucose and amino acids not diffuse through the phospholipid bilayer. or can ions such as sodium (Na+) or chloride (Cl ).

These can only cross the membrane with the help of certain protein molecules. channel proteins and carrier proteins

channel protein?

a membrane protein of fixed shape which has a water-filled pore through which selected hydrophilic ions or molecules can pass by facilitating diffusion or active transport

"gated" channel proteins?

This means that part of the protein molecule on the inside surface of the membrane can move to close or open the pore, like a gate. This allows control of ion exchange.

some channels occur..

in a single protein; others are formed by several proteins combined. Some gated channel proteins require energy (in the form of ATP) to operate the gate

carrier protein?

a membrane protein which changes shape to allow the passage into or out of the cell of specific ions or molecules by facilitated diffusion or active transport

how do carrier proteins allow facilitated diffusion?

The binding site is alternately open to one side of the membrane, then the other. This allows the molecule or ion to cross the membrane. Some carrier proteins change shape spontaneously.

pumps?

Some carrier proteins, known as pumps, require energy and are involved in active transport

the rate of diffusion through channel and carrier proteins depend on?

If molecules are diffusing across a membrane, the direction of movement depends on their relative concentration on each side of the membrane.

However, the rate at which facilitated diffusion takes place is also affected by how many channel or carrier protein molecules there are in the membrane and, in the case of channel proteins, on whether they are open or not.

osmosis?

the net diffusion of water molecules from a region of higher water potential to a region of lower water potential, through a partially permeable membrane

water potential?

: a measure of the tendency of water to move from one place to another; water moves from a solution with higher water potential to one with lower water potential; water potential is decreased by the addition of solute, and increased by the application of pressure; the symbol for water potential is ψ or ψw

when is equilibrium reached?

Water will move until the water potential is the same throughout the system

water potential depends on which two factors?

the concentration of the solution how much pressure is applied to it.

Pressure on a solution increases its water potential.

pressure units for water potential?

kiloPascals

kPa

assuming there's no extra pressure applied to the solution...

, the water potential of pure water will always be higher than the water potential of a solution

water potential of pure water?

0kPa

the water potential of all solutions must be ..

. Since all solutions have a lower water potential than pure water, the water potential of all solutions must be less than zero, their water potentials must be negative and the units will be -kPa.

the closer the water potential is to 0, the higher it is

if the water potential of the solution surrounding the cell is too high in animal cells..

the cell swells and bursts

if the water potential of the solution surrounding the cell is too low in animal cells...

the cell shrinks

protoplast?

the living contents of a plant cell, including the cell surface membrane but excluding the cell wall.

when water enters a plant cell by osmosis...

the volume of the cell increases but in the plant cell wall resists the expansion of the protoplast. Pressure rapidly starts to build up inside the cell. This pressure increases the water potential of the cell until the water potential inside the cell equals the water potential outside the cell, and equilibrium is reached.

why does it take very little water to enter the cell to achieve equilibrium?

The cell wall is so inelastic that the pressure builds up quickly

cell wall prevents cell from bursting

turgid cell?

when a plant cell is fully inflated with water

if a plant cell is placed in a concentrated solution...

water will leave the cell by osmosis and the protoplast gradually shrinks until it is exerting no pressure at all on the cell wall. Both the solute molecules and the water molecules of the external solution can continue to pass through the freely permeable cell wall, and so the external solution remains in contact with the shrinking protoplast.

plasmolysis?

the loss of water from a plant or prokaryote cell to the point where the protoplast shrinks away from the cell wall

Incipient plasmolysis

: the point at which plasmolysis is about to occur when a plant cell or a prokaryote cell is losing water; at this point the protoplast is exerting no pressure on the cell wall

active transport?

: the movement of molecules or ions through transport proteins across a cell membrane, against their concentration gradient, using energy from ATP.

potassium and chloride ions are often found to be ...

be 10-20 times more concentrated inside cells than outside. So a concentration gradient exists, with a lower concentration outside and a higher concentration inside the cell. The ions inside the cell originally came from the external solution. Therefore diffusion cannot be responsible for the gradient (high to low concentration). The ions must therefore build up against a concentration gradient.

the carrier proteins are called...

pumps, each of which is specific for a particular type of molecule or ion.

unlike facilitated diffusion, active transport requires...

energy because movement occurs up a concentration gradient rather than down. the energy is most often supplied by ATP. The energy is used to make the carrier protein change its shape, transferring the molecules or ions across the membrane in the process

sodium-potassium pump:

a membrane protein (or proteins) that moves sodium ions out of a cell and potassium ions into it, using ATP

the role of the sodium potassium pump is to..

pump three sodium ions out of the cell at the same time as allowing two potassium ions into the cell for each ATP molecule used. Sodium and potassium ions are both positively charged, so the net result is that the inside of the cell becomes more negative than the outside. A potential difference (p.d.) is created across the membrane

the pump has a receptor site for..

ATP on its inner surface.The receptor site acts as an ATPase enzyme in bringing about the hydrolysis of ATP to ADP (adenosine diphosphate) and phosphate to release energy

active transport can therefore be defined as..

the energy consuming transport of molecules or ions across a membrane against a concentration gradient

where is active transport important?

also involved in the absorption of some products of digestion from the gut.

In plants, active transport is used to load inorganic ions from the soil into root hairs

why have mechanisms evolved for the bulk transport of large quantities of materials into and out of cells?

Materials include large molecules such as proteins or polysaccharides, parts of cells or even whole cells

endocytosis?

the bulk movement of liquids (pinocytosis) or solids (phagocytosis) into a cell, by the infolding of the cell surface membrane to form vesicles containing the substance; endocytosis is an active process requiring ATP

phagocytosis or cell eating?

the bulk uptake of solid material. The process is called phagocytosis and the vacuoles are called phagocytic vacuoles.

pinocytosis or cell drinking?

the bulk uptake of liquid. The vacuoles or vesicles formed are often extremely small, in which case the process is called micropinocytosis.

phagocyte?

a type of cell that ingests (eats) and destroys pathogens or damaged body cells by the process of phagocytosis; some phagocytes are white blood cell.

exocytosis?

the bulk movement of liquids or solids out of a cell, by the fusion of vesicles containing the substance with the cell surface membrane; exocytosis is 1 an active process requiring ATP

examples of exocytosis?

the secretion of digestive enzymes from cells of the pancreas. Secretory vesicles from the Golgi apparatus carry the enzymes to the cell surface and release their contents. Plant cells use exocytosis to get their cell wall building materials to the outside of the cell surface membrane